In light sources of the above kind the radiation is emitted from hot plasma produced by a pulsed current. Very powerful EUV radiation generating devices are operated with metal vapor to generate the required plasma. An example of such a device is shown in WO2005/025280 A2. In this known EUV radiation generating device the metal vapor is produced from a metal melt which is applied to a surface in the discharge space and at least partially evaporated by a pulsed energy beam, in particular a laser beam. In a preferred embodiment of this device the two electrodes are rotatably mounted forming electrode wheels which are rotated during operation of the device. The electrode wheels dip during rotation into containers with the metal melt. A pulsed laser beam is directed directly to the surface of one of the electrodes in order to generate the metal vapor from the applied metal melt. This metal vapor cloud expands towards the second electrode and leads to a short circuit between the two electrodes which are connected to a charged capacitor bank, thus igniting the electrical discharge. Due to the low inductance of the electrical circuit, an electrical pulse with a few tens of kA is created that heats the plasma to several tens of eV within around 100 ns. Through this heating the desired ionization stages are exited and radiation in the EUV region is emitted from a pinch plasma. The conversion efficiency is defined as the ratio of EUV radiation, i.e. a 2% bandwidth centered on 13.5 nm, emitted in 2π sr and the energy initially stored at the capacitor bank.
For application of this EUV radiation in a EUV scanner not only the amount of EUV radiation produced per pulse is of interest, but also the fraction that can be used by the scanner. This holds only for the radiation originating from a sphere with around 1 mm diameter. The exact diameter depends on the solid angle of the collector optics and the étendue of the scanner. Especially for discharge produced plasmas, like the above of the so called “Aachener Lampe”, it is known that not all of the produced EUV radiation is centered within the above collectable volume. This is mainly due to the large region between the center of the plasma and one of the electrodes from which region EUV is emitted as well. In spite of the low intensity, the total amount of energy emitted from this region is still significant due to its large volume. The collectable conversion efficiency (CCE) is the ratio of collectable EUV radiation and the electrical pulse energy, and is therefore the metric for the overall efficiency of the EUV generation.